The present invention deals with lithium and lithium-ion batteries which exhibit surprisingly high capacity retention at high discharge rates and long cycle life, and with an elevated temperature melt process for the fabrication thereof.
It is known in the art to employ graphite current collectors in electrochemical cells, particularly in environments which present a risk of corrosion for metal collectors. The metal collectors, except for this potential corrosion, are preferred for their high current-carrying capability. In some cases noble metals are employed but their high costs preclude their use in most commercial applications. Most typically aluminum and copper are the materials of choice for current collectors in lithium and lithium-ion cells.
Toyuguchi et al., JP-A Sho 58(1983)-115777, discloses an artificial graphite plate current collector employed in a lithium metal/polyacetylene cell wherein a solution of LiClO4 in propylene carbonate or lithium tetrafluoroborate in gamma-butyrolactone is employed as the electrolyte. Charging efficiency is shown to be much improved over comparable cells having metal current collectors. Charging efficiency was determined utilizing charge-discharge cycles of 2 hours or longer, that is the currents employed represent no more than 50% of that required to discharge the cell in one hour. The cell employed by Toyuguchi is not a lithium-ion cell.
British Patent Specification 1,214,4123, to the Standard Oil Company, discloses the use of a flexible graphite sheet as a current collector embedded in a carbon electrode useful in a molten salt battery involving the use of binary salt electrolytes, primarily a combination of LiCl and KCl. The graphite sheet was of a thickness of ca. 250 xcexcm, and was characterized by electrical resistivity in the plane of the sheet of 8xc3x9710xe2x88x924 ohm-cm.
Gratzel et al., WO 99/59218, is drawn to a lithium or lithium-ion battery of which one electrode is composed of a solid material characterized by a mesoscopic morphology. Disclosed are secondary (rechargeable) cells wherein lithium bis(trifluorosulfonyl) imide or lithium tris(trifluorosulfonyl) methide may be employed as electrolytes in aprotic solvents. A cell comprising TiO2 as the anode and LiyMn2O4 as the cathode, a solution of lithium bis(trifluorosulfonyl) imide in methoxypropionitrile as the electrolyte solution, and a paper separator was found to sustain a 10C discharge rate and maintain a cell voltage of 1.5V. Current collectors are conducting indium tin oxide deposited on glass substrates. A TiO2 anode is preferred over a carbon anode.
Fujimoto et al., JP Hei 5 (1993)-290887, discloses a secondary lithium-ion cell comprising a cathode of a lithium metal oxide compound, a cathode current collector of a graphite powder composite with polypropylene resin, an anode of powdered graphite, an anode current collector of a copper foil, and an electrolyte solution of LiCF3SO3 dissolved in a mixture of ethylene carbonate and dimethyl carbonate. The battery cell so formed is said to provide improvements over the existing state of the art cells which employ LiPF6 electrolyte in conjunction with cathode current collectors of aluminum, the improvements being a reduction in explosion hazard in a short circuit, and the charge discharge characteristics improved. A cell having a capacity of 500 mA-h was discharged at 200 mA in the voltage range of 4.1 to 3 volts; a very high percentage of the capacity was utilized, and restored upon recharging.
The present invention provides for a lithium or lithium-ion electrochemical cell, the cell comprising an anode, an anode current collector in electronically conductive contact with said anode, a cathode exhibiting an upper charging voltage in the range of 3 to 5 volts with respect to a Li/Li+ reference electrode, said cathode comprising a lithium insertion transition metal oxide, phosphate, or sulfate in electronically conductive contact with a cathode current collector having a thickness of less than 250 micrometers comprising graphite said graphite being characterized by a bulk density in the range of 0.08-2.25 g/cc, an electrical conductivity of at least 500 Siemens/cm, and said electronically conductive contact being characterized by a resistance of less than 50 ohm-cm2; an ion-permeable membrane as a separator between said cathode and anode, and an electrolyte solution being in ionically conductive contact with said anode and cathode, the electrolyte solution comprising an aprotic polar solvent and a lithium compound at a concentration in the range of 0.2 up to 3 molar, said lithium compound being represented by the formula
Rf1SO2Xxe2x88x92(Li+)YZa
wherein X is C or N, a=0 or 1 with the proviso that a=1 when X is C and a=0 when X is N; wherein when a=1, Y and Z independently are electron-withdrawing groups selected from the group consisting of CN, SO2Rf2, SO2R, P(O)(OR)2, CO2R, P(O)R2, C(O)Rf3, C(O)R, cycloalkenyl groups formed therewith, and, H, with the proviso that Y and Z cannot both be H; wherein further Rf1, Rf2 and Rf3 are perfluoroalkyl radicals of 1-4 carbons optionally substituted with one or more ether oxygens; R is an alkyl group of 1-6 carbons optionally substituted with one or more ether oxygens, or an aryl group optionally further substituted; or wherein, when a=0, Y is an electron-withdrawing group represented by the formula xe2x80x94SO2Rf6 where Rf6 is the radical represented by the formula xe2x80x94(Rf4SO2Nxe2x88x92(Li+)SO2)mRf5 where m=0 or 1, and Rf4 is xe2x80x94CnF2nxe2x80x94 and Rf5 is xe2x80x94CnF2n+1 where n=1-4, optionally substituted with one or more ether oxygens.
The present invention further provides for a process for forming an electrochemical cell, the process comprising forming a melt processible composition by combining in a vessel provided with a mixing means a polymer, a mixture of one or more polar aprotic liquids, and a lithium compound; mixing said composition at least until it is plastically formable; and, forming a sheet from said plastically formable composition by the application of heat and/or pressure thereto; layering said sheet with a graphite current collector sheet having a bulk density of 0.08-2.25 g/cc, a thickness of less than 250 micrometers and a conductivity of at least 500 Siemens/cm and such other components as are required to make an electrochemical cell; and, consolidating said layered shaped articles so that the layers are in electrically and/or ionically conductive contact as necessary to form an electrochemical cell, said lithium compound represented by the formula
Rf1SO2Xxe2x88x92(Li+)YZa
wherein X is C or N, a=0 or 1 with the proviso that a=1 when X is C and a=0 when X is N; wherein when a=1, Y and Z independently are electron-withdrawing groups selected from the group consisting of CN, SO2Rf2, SO2R, P(O)(OR)2, CO2R, P(O)R2, C(O)Rf3, C(O)R, cycloalkenyl groups formed therewith, and, H, with the proviso that Y and Z cannot both be H; wherein further Rf1, Rf2 and Rf3 are perfluoroalkyl radicals of 1-4 carbons optionally substituted with one or more ether oxygens; R is an alkyl group of 1-6 carbons optionally substituted with one or more ether oxygens, or an aryl group optionally further substituted; or wherein, when a=0, Y is an electron-withdrawing group represented by the formula xe2x80x94SO2Rf6 where Rf6 is the radical represented by the formula xe2x80x94(Rf4SO2Nxe2x88x92(Li+)SO2)mRf5 where m=0 or 1, and Rf4 is xe2x80x94CnF2nxe2x80x94 and Rf5 is xe2x80x94CnF2n+1 where n=1-4, optionally substituted with one or more ether oxygens.